<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app">
        firstName:
        <input type="text" v-model="firstName">
        lastName:
        <input type="text" v-model="lastName">
        fullName:
        {{ fullName }}
    </div>
    <script src="./vue.js"></script>
    <script>
        let vm = new Vue({
            el:"#app",
            data:{
                firstName:"",
                lastName:""
              
            },
           computed:{
               fullName(){
                   return this.firstName+"."+this.lastName;
               }
           }
        })
    </script>
</body>
</html>